fix: deal with fast consecutive promise resolutions when streaming (#9332)

* fix: deal with fast consecutive promise resolutions when streaming

fixes #9330

* Update packages/kit/src/utils/streaming.js

Author: dummdidumm

.changeset/strange-garlics-pump.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: deal with fast consecutive promise resolutions when streaming

packages/kit/src/utils/streaming.js

@@ -23,22 +23,29 @@ function defer() {
  * }}
  */
 export function create_async_iterator() {
-	let deferred = defer();
+	let deferred = [defer()];
 
 	return {
 		iterator: {
 			[Symbol.asyncIterator]() {
 				return {
-					next: () => deferred.promise
+					next: async () => {
+						const next = await deferred[0].promise;
+						if (!next.done) deferred.shift();
+						return next;
+					}
 				};
 			}
 		},
 		push: (value) => {
-			deferred.fulfil({ value, done: false });
-			deferred = defer();
+			deferred[deferred.length - 1].fulfil({
+				value,
+				done: false
+			});
+			deferred.push(defer());
 		},
 		done: () => {
-			deferred.fulfil({ done: true });
+			deferred[deferred.length - 1].fulfil({ done: true });
 		}
 	};
 }

packages/kit/src/utils/streaming.spec.js

@@ -0,0 +1,20 @@
+import { test } from 'uvu';
+import * as assert from 'uvu/assert';
+import { create_async_iterator } from './streaming.js';
+
+test(`works with fast consecutive promise resolutions`, async () => {
+	const iterator = create_async_iterator();
+
+	Promise.resolve(1).then((n) => iterator.push(n));
+	Promise.resolve(2).then((n) => iterator.push(n));
+	Promise.resolve().then(() => iterator.done());
+
+	const actual = [];
+	for await (const value of iterator.iterator) {
+		actual.push(value);
+	}
+
+	assert.equal(actual, [1, 2]);
+});
+
+test.run();